Bank of India - Fraudulent transactions on my bank account

I am filing this complaint on behalf of my father - Mr Manchu Mandal. He has a joint savings account with my mother - Anuradha Mandal - in Bank of India (New Delhi - Malai Mandir Branch, R K Puram). There has been multiple fraudulent transactions on his account between * Oct, **** and ** Nov, **** totalling an amount of Rs *,**,***. Please note that he did not have any mobile number registered with the account, so there is no question of OTP generation and my parents giving it away to anyone. When approached the bank, they would file a insurance claim as the amount in the savings account are secured by their insurance. After multiple follow ups and waiting for an year, we were told that insurance has declined the claim.
Now we are not sure how to handle this. Please help us with sending any legal notice to the bank and get our money.
We did file the complaint back in **** with local police station, where they did general diary. We also filed a cyber cell complaint online in ****, but that also did not result in any positive result. Attaching all the details for your reference.